Question

Why the reducing character of trihydrides of group $15$ elements increases down the group?

Answer 1

Trihydrides of group 15 elements are given on the basis of reducing character as following

$Bi{H}_3>Sb{H}_3>As{H}_3>P{H}_3>N{H}_3$

The reducing character increases down the group because the bond strength of the $E-H$ bond decreases.

As the size of the element $E$ down the group increases and thus the bond dissociation energy decreases. This facilitates the dissociation of Hydrogen atoms and hence down the group, thus the reducing character increases.